Loss of relativistic electrons: Evidence for pitch angle scattering by electromagnetic ion cyclotron waves excited by unstable ring current protons

Abstract

[1] During geomagnetic storms the flux of radiation belt electrons can increase, decrease, or stay constant, depending on the competition between acceleration and loss mechanisms. We focus on loss of relativistic electrons. We use low-altitude polar-orbiting spacecraft and analyze fluxes of tens to hundreds of keV protons and relativistic (>1.5 MeV) electrons during a moderate geomagnetic storm, with a long-lasting recovery phase (4-5 d). Using data from four local times, we find that the loss of relativistic electrons is confined within the anisotropic proton zone and that a spatially limited loss of relativistic electrons is spatially collocated with increased loss of protons. The proton pitch angle distributions within these peaks are consistent with moderate to strong pitch angle scattering due to electromagnetic ion cyclotron (EMIC) waves. The loss of relativistic electrons collocated with protons is found at all four local times considered (0300, 0700, 1400, 1700 MLT). Since anisotropic proton distributions can under certain conditions generate EMIC waves, we find strong indications that the observed relativistic electrons are scattered into the atmospheric loss cone by EMIC waves. EMIC wave scattering is less efficient at high equatorial pitch angles but very efficient near the loss cone, thereby controlling the loss rate of relativistic electrons to the atmosphere. Our observations in and near the loss cone support theoretical work suggesting that EMIC waves can cause scattering loss to the atmosphere of relativistic electrons over the course of a geomagnetic storm.Citation: Sandanger, M., F. Søraas, K. Aarsnes, K. Oksavik, and D. S. Evans (2007), Loss of relativistic electrons: Evidence for pitch angle scattering by electromagnetic ion cyclotron waves excited by unstable ring current protons,

Relativistic electron losses related to EMIC waves during CIR and CME storms

“…The prolonged substorm activity (that gives us a prolonged recovery phase) produces enhanced whistler mode chorus and electromagnetic ion-cyclotron (EMIC) waves. The whistler mode chorus will accelerate electrons to relativistic energies while EMIC waves will scatter the particles into the atmospheric loss cone (Summers et al, 1998(Summers et al, , 2004(Summers et al, , 2007aSummers and Ma, 2000;Sandanger et al, 2007). Sandanger et al (2007) investigated atmospheric loss of relativistic electrons during geomagnetic storms.…”

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

“…A number of studies showed the evidence that the loss of relativistic electrons in the radiation belts is caused by EMIC wave scattering (e.g., Millan et al 2007;Sandanger et al 2007;Rodger et al 2008;Miyoshi et al 2008;Blum et al 2012;Carson et al 2013). Shprits et al (2013) demonstrated that EMIC waves play a crucial role for the dynamics of the ultrarelativistic populations in the radiation belts.…”

Model of electromagnetic ion cyclotron waves in the inner magnetosphere

“…EMIC waves are most easily generated in the vicinity of the magnetic equator with quasi-field-aligned wave normal angles, and they have frequencies below the proton gyrofrequency [Kennel and Petschek, 1966]. These waves strongly affect the RC protons [Gonzalez et al, 1989], thermal electrons [Cornwall et al, 1971], thermal/suprathermal ions [Anderson and Fuselier, 1994;Fuselier and Anderson, 1996;Horne and Thorne, 1997], hot heavy ions Horne, 1994, 1997], and the outer radiation belt (RB) relativistic electrons [Lorentzen et al, 2000;Sandanger et al, 2007], leading to nonadiabatic particle heating and/or pitch angle scattering in the eV-MeV energy range.…”